How Do We Know What It How Do We Know What It Is Like Inside The Earth?Is Like Inside The Earth?

Pre-GPT scientific thought on the Earth before Pre-GPT scientific thought on the Earth before 19501950

Department of Geological SciencesDepartment of Geological SciencesSan Diego State UniversitySan Diego State University

What direct evidence tells you aboutWhat direct evidence tells you aboutthe inside of the Earth?the inside of the Earth?

How Symmetrical Is the Earth?How Symmetrical Is the Earth?

How Symmetrical Is the Earth?How Symmetrical Is the Earth?Very, a the rotation about its axis is Very, a the rotation about its axis is

fairly smoothfairly smooth

Any reasonableEarth

model proposed must be

symmetrical

Symmetry would be imposedby initial spinning

while molten

The Interior Temperature of the EarthThe Interior Temperature of the Earth

Is the inside of the Earth hot or cold?Is the inside of the Earth hot or cold? What evidence is there for this?What evidence is there for this?

Evidence that the Earth is hot inside:Evidence that the Earth is hot inside:

VolcanoesHot Geysers

Hot sulfurous gasses from vents

Deep MinesDeep Mines

Evidence that the Earth is hot inside:Evidence that the Earth is hot inside:

The temperature in all deeper mines increases with depth: About 30 oC per 1000 m

The limit to the depth ofa deep mine is whenthe temperature is to

hot for stand

This image is in a SouthAfrican Gold mine 3km or2 miles deep in the Earth.The temperature is 49 C or 120 F

Deepest mine is 3,585 m or 11,762 ft

EarthRadius~6,400 km

.

DeepestMine

3.4 km

Deepest mines only extend a tiny fraction into the average thickness the crust, never mind the rest of the Earth

What About Magnetism?What About Magnetism?

Is the Earth’s magnetic?Is the Earth’s magnetic? What sort of properties does it have?What sort of properties does it have? Could you detect the Earth’s Could you detect the Earth’s

magnetic fieldmagnetic field

The Earth’s The Earth’s Magnetic Field: by Magnetic Field: by

compasscompassAge of Exploration, 1400-1600,also discovered the compass andEarth’s magnetic field.

Magnetic Dip:Magnetic Dip:the angle that magnetic the angle that magnetic

lineslinesleave or enter leave or enter

the Earththe Earth

Aurora Borealis and Aurora Australis,Aurora Borealis and Aurora Australis,northern and southern lightsnorthern and southern lights

Most frequent is near 67 N and S latitudeMost frequent is near 67 N and S latitude

MaximumN. Pole Coverage

MaximumS. Pole Coverage

Earth’s magnetic field mustbe deep inside,And uniform and symmetrical to cause symmetrical compass variations andthe Auras Borealis and Australis

What can gravity tell usWhat can gravity tell usabout the Earth?about the Earth?

Sir Isaac NewtonPublished his

Philosophiae Naturalis Principia1686

which included the definitivestatement on gravitation

I amSo

proud

Basic Gravity:Basic Gravity:Any two bodies in the universe

attracted each other with a force that is

GREATER

if either is more MASSIVE

GREATER if they are CLOSER

Force, equal andopposite

Force, equal andopposite

Hey,this

Took a lot of work

Gravity: The FormulaGravity: The Formula

G

D

m1

m2

Memorizethis for the

quiz

Cavendish’s 1789 Experiment to Determine Cavendish’s 1789 Experiment to Determine the Mass and Density of the Earththe Mass and Density of the Earth

* Use torsion balance to measure the force between 4 iron balls* Measure the size and mass of the iron balls* Calculate “constant” in gravity formula

Earth

G

EarthRadius

Ballmass

Earthmass

Ballweight

Earth Mass = 6 x 10 24kg

EarthAverage Density

= 5.54 g/cm3

Cavendish’s 1789 Experiment to Determine Cavendish’s 1789 Experiment to Determine the Mass and Density of the Earth:the Mass and Density of the Earth:

What does this mean?What does this mean?

Earth

EarthAverage Density

= 5.54 g/cm3You

should see this on myFacebook

HenryCavendish~ 1789

Cavendish’s 1789 Experiment to Determine Cavendish’s 1789 Experiment to Determine the Mass and Density of the Earth:the Mass and Density of the Earth:

What does this mean?What does this mean?

Earth

EarthAverage Density

= 5.54 g/cm3

Surface rocks asBasalt, Granite

~ 3 g/cm3

Deeper Rocks must bemuch denser than 5.54 g/cm3

to compensate for lighter surface rocks

Water= 1 g/cm3

What is the What is the material at the material at the

center of the Earth? center of the Earth? Iron -Nickel Iron -Nickel

The Earth is thought to be similar to Meteorites, about 5% of the meteoritesare Iron-Nickel metal. These are believed to have been the centerof former bodies in space where the Iron-Nickel, a dense material, settled tothe center when the body was molten.A later collision broke apart the body into many small parts.

Iron-Nickel meteorite found in Oregon

Gravity is significantly affectedGravity is significantly affectedby masses close to the Earth’s surface by masses close to the Earth’s surface

– even only a kilometer or so under ground– even only a kilometer or so under ground

Earth’sRadius

6,500 km

Ore bodies near the surfaceOre bodies near the surfacewill cause gravity to be more and less –will cause gravity to be more and less –

a way of searching for thema way of searching for them

The BritishThe BritishGreat Arc Great Arc Meridian Meridian Survey of Survey of

India India 1803-501803-50

The British discovered that their The British discovered that their plumb bobs were not pointing plumb bobs were not pointing toward the center of the Earthtoward the center of the Earth

What could be an explanation?

Plum bobs, gravity and floatingPlum bobs, gravity and floating

Airy and Pratt proposed different explanations for the nature of the crust.

Airy hypothesized that there was a uniform density crust to the Earth and it varied in thickness – he was more right.

George Airy

John Pratt

Isostasy and Floating Analogy:The crust floats on the mantel. A plate floats on a portion of

mantle.

Figure 2-5

Crust, Mantle, Core, Plates all - floatCrust, Mantle, Core, Plates all - floatwith lighter on top, heavier below:with lighter on top, heavier below:

Just as objects do in waterJust as objects do in water

Principle of Isostacy: “A body that is less dense than a surrounding fluidwill displace a volume of the fluid equal to that body’s mass”

What Can Sound Tell Us?What Can Sound Tell Us?

Hint: Earthquakes are mostly sound energy

Earthquakes are used to probe the interior of theEarth, somewhat as a bat uses sound not only to detect the presence of objects but about the objects’ size and nature.

(Garrison, 1993)

• • Primary (P) waves:Primary (P) waves:- compressional- compressional- analogy is sound waves- analogy is sound waves

• • Secondary (S) waves:Secondary (S) waves:- transverse (analogy is ocean - transverse (analogy is ocean waves)waves)- ~1/2 velocity of P waves- ~1/2 velocity of P waves- do not pass through liquid- do not pass through liquid

• • By the way, surface waves (a By the way, surface waves (a

third type slower than above third type slower than above two) cause the big disasterstwo) cause the big disasters

Prediction forHomogeneous

Earth Structure

(Garrison-3.7Top)

(Garrison, 1993)

Observation from Global Seismological

Networks

(Pink Floyd, 1973)

..

(Pink Floyd, 1973)

Connect the dots: Then you get curves ofConnect the dots: Then you get curves oftemperature, density, P and S speed, temperature, density, P and S speed,

properties and moleculesproperties and molecules

Crust

The EndThe End